Do the model first, since the controller will depend on the model.
I did this exact same exercise when I was learning rails and it's a GREAT way to pick up both TDD and Rails because down the line you'll be able to compare your results against some of the login generators out there and see where you and the others differ.
It'll also give you a thorough understanding of your login system which sadly, if generated, can be a hole in someone's intellectual command of one of the most crucial pieces of their application.
For learning TDD I suggest checking out Geoffrey Grosenbach's peep cast on it: http://peepcode.com/products/test-first-development
Oh and also if you want a headstart on the login system Chad Fowler's Rails Recipes has some nuggets for both authentication and authorization.
I haven't viewed this particular peepcast but I've watched the others and they are excellent.
Good luck,
Tim Case tim@karmacrash.com